Oct 1 2002
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Several common types of defects arise due to the hot-dip galvanizing process. This article provides an explanation for the causes of defects and variations in appearance.
Defects in Galvanized Coatings
Ungalvanized Weld Areas
Coating misses on weld areas are the result of the presence of welding slag on the welds. The fabricator must remove all the welding slag before dispatch to the galvanizer.
Dark Staining Adjacent to Welds
Preparation chemicals that penetrate unsealed overlaps or via poor-quality welds boil out of the connection at the time of galvanization and cause coating misses and surface contamination during galvanizing.
Furthermore, anhydrous fluxing salts that remain in the connection will absorb atmospheric moisture and leach out onto the neighboring galvanized surface. Leaching of these salts will ultimately reach equilibrium. The affected area should be properly washed to eliminate moderately corrosive leachate.
Dull Gray or Mottled Coatings
Reactive steels will produce thicker galvanized coatings that are duller than regular coatings. These coatings possess longer life due to their greater thickness and their appearance is a function of steel metallurgy and usually beyond the control of the galvanizer.
Dross Pimples/Inclusions
Dross develops during the process of galvanization in the form of zinc-iron crystals (approximately 95% zinc and 5% iron) with a higher melting point compared to the metal in the zinc bath. Dross trapped in the galvanized coating may give the coating a gritty or rough look. The presence of dross additions in the coatings is not harmful to the coating’s performance as the corrosion resistance of zinc dross matches with that of the galvanized coating.
White Storage Staining
After galvanizing, items stacked or stored in wet, not well-ventilated conditions will react with atmospheric moisture to develop bulky white zinc hydroxide deposits on the surface of the galvanized coating.
Ash Staining
Zinc ash can develop during the galvanizing process as the work is immersed in the zinc. The ash formed is skimmed off the surface of the molten zinc before removing the work from the galvanizing bath. Occasionally, ash is trapped inside unreachable areas and sticks to the outer side of the coating as the work is removed from the bath. Ash may cause a light brown stain or a dull surface appearance after removal. It does not have any impact on the performance of the galvanized coating.
Striations and General Surface Irregularities
Ridges and lines thicker than the nearby galvanized coating are caused by varying rates of reaction of the zinc with the steel surface because of stress areas on the steel surface or the presence of weld areas or weld metal with altered metallurgy to the parent metal. This occurrence is most frequently seen on tube and pipe products. There is no impact on coating performance.
Runs, Drainage Spikes, and Puddling
These defects cannot be avoided during the hot-dip galvanizing of regular items and are tolerable as long as they do not affect the assembly of the function of the item or pose a safety risk in handling or service.
Bare Patches
Uncoated areas on the surface of galvanized work are because of not preparing the surface properly, pickling, insufficient pretreatment in degreasing, and pre-fluxing. It is necessary for these areas to be repaired using a suggested repair technique or the item should be regalvanized if the flaw is of considerable size.
Rust Staining
Uncoated steel exposed to galvanized coatings will quicken corrosion of the coating and stain the coating brown where contact occurs. Wire brushing will need to be done to remove it.
Delamination
Very heavy galvanized coatings (more than 250 µm in thickness) may be fragile and delaminate from the surface upon impact and need more cautious handling in transport and erection. Thin, cold-rolled items having a very smooth surface finish and made from reactive steel may also cause coating delamination.
Black Spots
Scattered black spotting is caused by residual galvanizing flux crystallizing on the surface of the work and is mostly because of flux contaminated rinse water or poor rinsing after galvanizing. This flaw is typically seen in galvanizing baths using the “wet” galvanizing process where the flux is on top of the molten zinc. Surplus aluminum in the galvanizing bath can also be the reason for this defect.
Spangled Coatings
Certain hot-dip galvanized coatings display a high level of “spangling” brought about by zinc crystal patterns on the surface. This is the case with galvanizing alloys created in specific smelting processes and these alloys are usually used for hot-dip galvanizing. The coating performance remains unaffected.